Eliminate FSFS format 7 CPU overhead from svnadmin.

This introduces a simply threshold value for the cache sizes above
which we will enable block-read.

While svnadmin is the ideal application for "block-read" (read all
repo data somewhat linearly and use virtually all of it), the very
small default cache sizes are simply not enough to benefit from it.
As a result, hit rates drop even further.

For larger caches, the feature equalizes most of the structural
disadvantage that packed f7 repos have over f6 (f6 is in revision
order and svnadmin is the only tool to read it in that ordering).
For smaller caches, only meta data and certain temporary objects
get cache while we heavily rely on the OS disk cache to provide
us with data.  This is equally true for all FSFS formats.

* subversion/svnadmin/svnadmin.c
  (BLOCK_READ_CACHE_THRESHOLD): Declare and document the new threshold.
  (open_repos): Set the FS config option for "block-read" depending
                on the configured membuffer cache sizes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1607306 13f79535-47bb-0310-9956-ffa450edef68